Solvent Trapping Effect on Pull-off Adhesion of Model Epoxy-amine Surface Coating

摘要

Pull-off peak stress is known to be thickness dependent, decreasing as a spontaneous delamination thickness is approached. Increase in film thickness adds to the total force acting on the interface in opposition to adhesion. When solvent is present, however, numerous layers of complication are added. A combination of applied film thickness, vitrification, and substrate induced anisotropic solvent loss results in solvent trapping that varies linearly with dry film thickness causing a cascade of other variables to do the same, e.g., modulus, work of adhesion, practical adhesion and internal stress. Ramifications from this work extend from formulator in terms of solvent selection, polymer chemistry, and kinetics, to applicator in terms of applied film thickness, substrate sorption, and layering, and field tester in terms of results versus thickness and age. Ideally, the solvents aiding in coating application evaporate from the film completely, but in reality, where ambient cure profiles are common, this is an unrealistic expectation and the elongated evaporation process extends from idealized minutes to months or years. Adhesion is the primary focus of this research based upon the importance of adhesion to coating performance. Specifically, pull-off adhesion is measured because of its relevance in the applied field. The data reveals a non-linear pull-off adhesion response to increasing thickness. In the presence of solvent, the negative relationship between pull-off peak stress and thickness becomes positive at a critical thickness. It is hypothesized that the inversion point corresponds to the thickness where stress relaxation due to solvent plasticization is greater than stress increase due to thickness. The most important connection to be made from the data appears to be that between solvent plasticization in comparison with thermal plasticization and the temperature dependence of adhesion. Results identified that peak adhesion for a given film was consistently at a temperature 20 °C below T_g prompting the hypothesis that when trapped residual solvent plasticizes a given coating material, the concentration of trapped solvent at the substrate interface equilibrates to a concentration that consistently results in a plasticized state with a T_g at 20 °C above the test conditions, i.e., peak adhesion results. The measured pull-off adhesion values for solvent-applied films were consistently greater than the bulk applied films, increasing by an average of over threefold at 50 μm, (just below the critical thickness). Additionally, this connection is supported by the solvent independence of the adhesion-thickness relationship in rubbery coatings. Knowing the residual solvent content varies with time and conditions of application, annealing, handling, cure, and lifetime, these results bring new understanding as to why adhesion values vary so dramatically for practical grounds, and yet are often not representative of measured real-world performance.